THE ORIGIN OF THE PERMO-TRIASSIC GAS ACCUMULATIONS IN THE IRANIAN ZAGROS FOLDBELT AND CONTIGUOUS OFFSHORE AREAS: A REVIEW OF THE PALAEOZOIC PETROLEUM SYSTEM
M. L. Bordenave*
* Consultant, 24 Ave. President Kennedy, 75016 Paris, France. email: Max.Bordenave@wanadoo.fr
More than 1500 trillion cubic feet (Tcf) of gas reserves have been discovered in Permo-Triassic carbonates sealed by thick Triassic anhydrites in the Zagros Foldbelt (SW Iran), the southern part of the Gulf (Iran, Qatar, and Abu Dhabi) and Saudi Arabia. This paper discusses the origins of this gas in terms of source rock distribution and thermal maturation through time (as indicated by modelling), regional variations in thermal maturation (as indicated by cumulative isopachs), and long-range migration and accumulation of hydrocarbon prior to the Zagros orogeny. The sequence of events leading to the present-day distribution of gas accumulations is reconstructed in detail.
The only important source rocks so far identified in the Late Proterozoic to Late Triassic succession are organic-rich, radioactive shales which are dated as Llandoverian (early Silurian). Oil generation began in the Middle Jurassic in areas of greatest subsidence, while the gas window was reached locally as early as the Middle Cretaceous. Huge volumes of oil, then of gas, accumulated in a few major regional highs and salt-related structures prior to the Zagros orogeny. Part of the gas was lost during Zagros folding as some of the anticlines were breached, and another portion, possibly associated with light oil, remigrated into unbreached Zagros anticlines.
Among critical parameters essential to the appraisal of the numerous Permo-Triassic prospects present in Lurestan, Fars and in the Iranian Offshore, three are discussed in this paper, namely (i) the location of prospects in comparison to pre-Zagros regional highs and to reconstructed pre-Zagros gas accumulations; (ii) the characteristics of potential reservoir intervals in the Dalan/Kangan Formation; and (iii) the extent of the Dashtak evaporitic seal. The distribution of surface oil, bitumen and gas seepages together with indirect hydrocarbon indications provides an additional exploration tool.
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